1. Field of the Invention
This invention relates to a breath-synchronized concentrated-oxygen supplier, and more particularly to an apparatus which supplies oxygen-enriched gas to a patient in synchronism with the inhalation phase of his respiration.
2. Related Art Statement
With the recent progress in the medical art, an increasing number of oxygen concentrators have been used in inhalation therapy for patients suffering from respiratory ailment or circulatory diseases. Particularly, oxygen concentrators for home use have become remarkably popular these days, because they are capable of concentrating the oxygen gas in air by the use of a household electric power source through a simple operation, and supplying such concentrated oxygen gas for medical use. In the United States of America, a standard for the home oxygen concentrator, namely American National Standard Institute (ANSI) Z79. 13, 1981, has been established under the guidance of Federal Food and Drug Administration (FDA). Further, an international standard for it, e.g., International Organization for Standardization ISO 5059, is now ready to be published. In countries where medical treatment at home prevails, such oxygen concentrators are used to eliminate the inconvenience involved in the conventional oxygen distribution by use of heavy pressure vessels such as gas cylinders.
In general, there are two kinds of methods for treating patients by inhalation of oxygen gas or the like; namely, the so-called "closed circuit method" and the so-called "open circuit method".
The closed circuit method uses a "facemask apparatus" on the face or an endotracheal tube inserted into the trachea of the patient and supplies the gas to the patient through a passage, which is airtightly separated from the atmosphere and extends between the breathing device, i.e., the respirator or the gas supply system, and the patient's respiratory system. This closed circuit method has an advantage in its high inhaling efficiency, because the gas can be inhaled at about the same concentration as that of the gas supplied to the closed passage and the patient's breathing can be assisted or adjusted by regulating the pressure of the gas inside. However, the closed circuit method has a shortcoming in that it may cause irritation or discomfort on the side of the patient because of the covering of his mouth and nose and the direct insertion of a foreign substance in his trachea. Accordingly, the closed circuit method has been used mainly for seriously sick and unconscious patients or patients under anesthesia.
On the other hand, the open circuit method uses a breathing passage which is open to the atmosphere. In this method, the tip of the gas supplying tube is inserted into the nostril or the mouth of the patient so as to feed the gas without using any airtight connection between the apparatus and the face or upper airway of the patient, and the irritation or discomfort on the side of the patient are reduced and the patient is allowed to speak, eat or drink during the inhalation treatment by this method. Accordingly, this open circuit method is mainly used for mild cases in which self-breathing is possible.
In the closed circuit type breathing apparatus, oxygen or gas mixture for inhalation may be supplied in response to the patient's spontaneous breathing by detecting the gas pressure changes in the closed respiratory circuit, because the latter can be used as a triggering mechanism. However, in the conventional open circuit type breathing apparatus, it is difficult to detect a pressure change which is large enough to trigger the gas supply in the open respiratory circuit, and in most cases, the gas is supplied at a constant flow rate regardless of the patient's breathing. Accordingly, the gas is forced to the patient even during his exhalation and discomfort has been caused to the patient. Besides, a large part of the constantly fed gas is wasted because the gas supplied during patient's exhalation is discharged to the atmosphere without being used.
Besides, the open circuit type breathing apparatus is susceptible to undue dilution of oxygen concentration with air because it is open to the atmosphere. To cope with such partial pressure reduction of oxygen, it has been the practice to increase the flow rate of the constantly fed gas. However, the inventors have found that the transcutaneous tissue partial pressure of oxygen (tcPO.sub.2) increases with the flow rate only up to 3 l/min, and the oxygenation in vivo hardly further increases even when the oxygen flow rate of insufflation exceeds the above value, as shown in Table 1.
TABLE 1 ______________________________________ Transcutaneous Tissue Oxygen Partial Pressure for Different Constant Oxygen Flow Rates through a Nasal Cannula Flow rate (l/min) 0.5 1.0 2.0 3.0 4.0 ______________________________________ Oxygen partial 89.9 .+-. 93.1 .+-. 129.7 .+-. 145.2 .+-. 151.0 .+-. pressure 3.4 4.5 13.9 5.8 4.8 (mmHg) ______________________________________
Thus, when the oxygen flow rate is excessively high in such an open circuit type breathing apparatus, a large amount of the oxygen gas will be wasted to the atmosphere without being used by the living body. Furthermore, with a high flow rate of oxygen, the stimulation to the patient becomes too strong and patient's discomfort increases. Accordingly, there is a limitation in the constant flow rate oxygen insufflation system in clinical practice.
To overcome the shortcomings of the conventional open circuit type breathing system, the Japanese Patent laid-open Publication No. 8,972/84 proposed a breath-synchronized open circuit type breathing system. In the breath-synchronized type, oxygen gas is supplied only during inhalations of the patient, so that this type breathing system has advantages in that the patient's comfort is ensured during the inhalation treatment and that the oxygen concentrator can be made small due to the reduced use of oxygen.
The oxygen concentrators can be classified into two types, i.e., the so-called membrane type and the so-called molecular adsorption type. The membrane type oxygen concentrator passes the air through a special membrane which transmits oxygen more easily than nitrogen, so that the oxygen concentration is enhanced by increasing the number of oxygen molecules relative to the number of nitrogen molecules. With this membrane type, the maximum attainable oxygen concentration is limited to about 40% at most. Thus, the membrane type concentrator is rather suitable for closed circuit type breathing system in which the oxygen gas is inhaled at about the same concentration as supplied by the oxygen concentrator.
In the molecular adsorption type (also referred to as "pressure swing adsorption" type), the air is passed through an adsorption cylinder filled with a special substance (adsorbent) while increasing and decreasing the air pressure, and nitrogen and moisture in the air are removed by repeated adsorption and desorption processes so as to produce highly concentrated oxygen. With this type concentrator, an oxygen concentration higher than 90% can be obtained. Thus, the molecular adsorption type concentrator is suitable for long inhalation treatments by using an open circuit type breathing system allowing the mixing of open air with the concentrated oxygen gas for inhalation.
However, the molecular adsorption type has a shortcoming in that when the outflow of the oxygen-enriched gas increases, the amount of purge gas for regenerating the adsorbent decreases, resulting in a gradual reduction of the oxygen concentration of the oxygen-enriched output gas. Such reduction of the oxygen concentration is contrary to the very purpose of the oxygen concentrator. As a countermeasure, it has been tried to use oxygen concentrators of larger size and to improve their technical performance, but there have been certain limits in such trial.
To solve such shortcoming of the molecular adsorption type oxygen concentrator, Japanese Patent Application Publication No. 5,571/82 corresponding to U.S. Pat. No. 4,331,455 proposes an oxygen concentrator using two adsorption cylinders, which cylinders are alternately operated in such a manner that during the adsorption cycle of one cylinder, a part of the oxygen-enriched output gas from that cylinder is used as the purge gas for the other cylinder. Such oxygen concentrator with the two adsorption cylinders has an advantage in that, even when adsorption cylinders of comparatively small capacity are used, oxygen-enriched gas with a desired concentration can be produced over a long period of time with a high stability because the two cylinders are efficiently purged with each other.
The respiration pattern of a human being or the like living body will be briefly reviewed now. The oxygen partial pressure in the arterial blood during respiration can be effectively increased by providing a sufficiently high peak flow rate of oxygen at the beginning of the inhalation phase. The inhaled gas at the end portion of the inhalation phase does not reach the respiratory organ but fills up the so-called dead space portion, so that it is not used effectively in the respiratory organ. In view of the above characteristics of the respiration pattern, the efficiency of the oxygen-enriched gas usage in terms of its utilization factor can be improved by using such breath-synchronized control in which a sufficiently high peak flow rate of oxygen-enriched gas is superposed onto the initial portion of the steady state flow rate of such gas during the inhalation phase while the oxygen-enriched gas supply is interrupted at a certain end portion of the inhalation phase.
The breath-synchronized open circuit type breathing system disclosed by the above-mentioned Japanese Patent Laying-open Publication No. 8,972/84, however, uses such control that a constant flow rate of the oxygen gas with a certain concentration is maintained during the inhalation phase and the interruption of the oxygen gas supply near the end of the inhalation phase is effected by a one-shot circuit which is actuated at the beginning of the inhalation phase, so that the oxygen is supplied for a predetermined period of time. Thus, with this breathing system, the oxygen partial pressure of the blood may not be raised so effectively and the utilization rate of the oxygen gas may not be sufficiently high. Further, the duration of oxygen gas supply for the inhalation phase is set at a certain value but is not variable in response to the patient's respiration, so that the breathing system cannot respond well to irregularity of the respiration and it may sometimes become out of synchronism with the patient's respiration, resulting in a still lower utilization rate.
The respiration pattern of a human being inherently varies from person to person, and even for one person, the speed and magnitude of the respiration vary depending on circumstances. Even under the same conditions, actual measurements of the durations of individual inhalations and exhalations show dispersions. Ideally, the timing and duration of the oxygen gas supply from the breathing system should be automatically controlled so as to be in synchronism with each of the ever varying inhalation timing and duration depending on the personal, circumstantial and individual respiratory differences.
The oxygen concentrator disclosed in the above-mentioned Japanese Patent Laying-open Publication No. 5,571/82 produces an almost constant flow rate of the oxygen gas with a certain concentration, so that it has shortcomings in that its utilization rate in terms of usage by the living body may be relatively low and that it may still cause irritation and discomfort on the side of patients or the like.